Some image forming apparatuses use an endless belt to print an image on a sheet. There are also some image forming apparatuses that form toner images with different colors on a plurality of respective image bearing members and transfer the images on a sheet to print a color image on the sheet. Such a system is sometimes called tandem printing. Because such image forming apparatuses overlay toner images with different colors to transfer them on a sheet, they may use an endless belt as an intermediate transfer belt or a sheet transport belt. The belt is typically stretched around a plurality of rollers to be rotated. Unfortunately, the belt may suffer from skew resulting from factors such as the accuracy of manufacturing and mounting of the rollers, deviation of the axis of each roller from parallelism, nonuniformity in the thickness of the inner wall of the belt, or nonuniformity in the tension of the belt. If the skew of the belt is large, it may affect formation and transfer of a toner image. If the skew of the belt is large and the belt is too far to one side, the belt may be broken.
An image forming apparatus capable of determining the amount (degree) of skew of a belt has been proposed. The image forming apparatus includes a belt that conveys a sheet and detection means for detecting the position of a mark on the belt for use in determining the position in the belt width direction. In this image forming apparatus, the position of the belt in an initial state before the belt is driven is detected by the detection means detecting the mark, the detected position is stored, the position of the mark after a pass of a predetermined amount of the belt is detected by the detection means, and the detected position is stored. Then, the amount of skew of the belt is calculated using the difference between the position of the mark in the initial state and its position after the pass. With such a configuration, the initial speed of skewing (speed of skewing when the belt starts skewing) is detected.
To determine the state of skew of the belt, it is necessary to determine the position of the belt (position of the end of the belt) in the width direction (axial direction of the rollers). Thus, the image forming apparatus may include a detector for detecting the position of the edge of the belt. An output value (reference output value) of the detector when the belt is in a reference position (ideal position) in the axial direction of the rollers (reference output value) is determined in advance. If there is a difference between a current output value and the reference output value, the belt can be identified as being displaced from the reference position; if the difference between the output values varies, the belt can be identified as skewing.
To detect the position of the belt in the circumferential direction and measure the time required for the belt to make a single rotation, the belt may be provided with an index that projects from the edge of the belt. The position of the belt in the circumferential direction may be determined by detection of the index. The detection of the index enables optimizing the timing of forming an image, the timing of conveying a sheet, and other timings.
The position of the edge of the belt and the index may be detected by the use of an output value of a single detector (sensor). In other words, the position of the belt (amount of a skew) and the arrival or passage of the index may be detected by the use of a single sensor. However, because the index projects from the edge of the belt, if control relating to skew correction is performed on the basis of a variation in the output value of the sensor caused by an arrival or a passage of the index, then the control relating to skew correction in which the position of (the edge of) the index is viewed as the edge of the belt is performed. Thus, the skew correction may be larger than required, and the belt may further skew.
When the position of the edge of the belt and the arrival and passage of the index are detected by a single sensor, a threshold for the amount of change in the output value of the sensor is set to prevent incorrect skew correction and accurately determine the time of the arrival of the index and the time of the passage of the index. The control relating to skew correction (skew feedback control) is performed during the time the belt is driven. However, if the amount of change in the output value of the sensor is larger than the threshold, the image forming apparatus determines the passage or arrival of the index and stops the control relating to skew correction. If the amount of change in the output value of the sensor does not exceed the threshold, the image forming apparatus determines that the change in output value results from the skew of the belt and performs the control relating to skew correction.
However, the instantaneous maximum amount of the skew of the belt (maximum speed of the belt moving in the axial direction of the rollers) differs in accordance with the assembly accuracy and the precision of the members and varies from one apparatus to another. Some individual apparatuses (image forming apparatuses) include a belt that easily skews and have a large amount of change in the output value of a sensor during skewing. Other individual apparatuses include a belt that does not easily skew and have a small amount of change in the output value during skewing. There are also variations in the output characteristics among sensors. Accordingly, when the threshold of detection of the index is a value common to the model of an image forming apparatus (uniform threshold), a change in the output value of the sensor caused by the skew of the belt may be incorrectly detected as an arrival or a passage of the index. If incorrect detection relating to the index occurs, an error (imperfection) may occur in the timing of image formation or in the conveyance of a sheet. That may cause inconveniences, such as additional consumption of toner and sheets and necessity of tasks and operations for eliminating incorrect image formation and sheet conveyance.
In the above-described image forming apparatus, by detecting a formed toner image with the detection means, the amount of skew is detected. For detecting the position of the belt in the circumferential direction, the belt includes a belt hole, and a passage of the belt hole is detected by a home position sensor. That is, two items are detected by two sensors. Accordingly, even with the above-described image forming apparatus, when it detects the position of the edge of the belt and the position of the belt in the circumferential direction (index) with a single sensor, a problem remains unsolved that a change in the output value of the sensor caused by skew of the belt is incorrectly detected as an arrival or a passage of the index.